1. Field of the Invention
The present invention relates to a driving force distribution control system for controlling a distribution of a driving force from an engine between two drive members via a clutch that can be tightened or released, such as, a transfer clutch or a limited slip differential clutch, and to a vehicle having such driving force distribution control system.
2. Description of the Related Art
In general, many vehicles adopt a hydraulic multi-disc clutch or other clutches to control a distribution of a driving force to wheels. Specifically, the distribution of the driving force for a vehicle is controlled by controlling the distribution of the driving force to a front axle and/or a rear axle in a four-wheel drive vehicle, or by controlling a limited slip differential that connects or disconnects the front axle to or from the rear axle, or a left wheel to or from a right wheel.
When the distribution of the driving force is thus controlled, a clutch tightening force is determined accurately based on driving conditions of the vehicle. Thus, the distribution of the driving force is achieved properly without occurrence of tight corner braking phenomena due to a difference of a turning radius of wheels.
For example, Japanese Unexamined Patent Publication No. 8-2278 has disclosed a driving force control method for preventing an occurrence of the phenomena of the tight corner braking. According to this driving force control method, it is judged whether a four-wheel drive vehicle has made a turn at a large steering angle. If it is judged that the vehicle has made the turn at the large steering angle, a limited slip differential clutch is controlled in order to reduce a transmission torque.
However, there are many drawbacks described below in the foregoing method of controlling the driving force using a hydraulic multi-disc clutch. Namely, even when a transmission of the driving force is controlled as properly as usual, the phenomena of the tight corner braking may occur because of a difference in the properties of the clutch, a slight allowance difference of the product, and/or due to long use. Otherwise, main driving wheels may slip, or inner wheels alone may slip during turning of a vehicle.
Actually a transmitted torque may be detected and fed back in order to optimize a set value of clutch tightening force. In this case, however, a torque sensor must be installed on the driving axles. This method is therefore unfeasible (not practical).
Accordingly, an object of the present invention is to provide a driving force distribution control system for a vehicle. The driving force distribution control system can determine an appropriate clutch tightening force highly and precisely in consideration of a difference in the properties of the clutch, a slight allowance difference of the product, and/or due to long use without the necessity of mounting a torque sensor or the like on driving axles.
In efforts to accomplish the above object, according to the present invention, there is provided a driving force distribution control system for controlling a transmission of a driving force from a first driving axle to a second driving axle via a clutch, which comprises: basic clutch tightening force calculating means for calculating a basic clutch tightening force to be exerted in tightening the clutch; basic clutch tightening force correcting means for correcting the basic clutch tightening force by using a correction value; actual speed difference calculating means for calculating an actual speed difference between an actual revolution speed of the first driving axle and an actual revolution speed of the second driving axle; reference speed difference calculating means for calculating a reference speed difference in dependency on a kinetic state of a vehicle; and correction value setting means for correcting the correction value to be used by the basic clutch tightening force correcting means according to the actual speed difference and the reference speed difference.